Glossary of Irrigation Terms Version 7/1/17 Edited by Eugene W. Rochester, CID Certification Consultant This document is in continuing development. You are encouraged to submit definitions along with their source to [email protected]. The terms in this glossary are presented in an effort to provide a foundation for common understanding in communications covering irrigation. The following provides additional information: • Items located within brackets, [ ], indicate the IA-preferred abbreviation or acronym for the term specified. • Items located within braces, { }, indicate quantitative IA-preferred units for the term specified. • General definitions of terms not used in mathematical equations are not flagged in any way. • Three dots (…) at the end of a definition indicate that the definition has been truncated. • Terms with strike-through are non-preferred usage. • References are provided for the convenience of the reader and do not infer original reference. Additional soil science terms may be found at www.soils.org/publications/soils-glossary#. A AC {hertz}: Abbreviation for alternating current. AC pipe: Asbestos-cement pipe was commonly used for buried pipelines. It combines strength with light weight and is immune to rust and corrosion. (James, 1988) (No longer made.) acceleration of gravity. See gravity (acceleration due to). acid precipitation: Atmospheric precipitation that is below pH 7 and is often composed of the hydrolyzed by-products from oxidized halogen, nitrogen, and sulfur substances. (Glossary of Soil Science Terms, 2013) acid soil: Soil with a pH value less than 7.0. (Glossary of Soil Science Terms, 2013) adhesion: Forces of attraction between unlike molecules, e.g. water and solid. (Glossary of Soil Science Terms, 2013) adjusted sodium adsorption ratio [adj. RNa]: Index of permeability problems, based upon water quality. (Burt, 1998) adsorption: The process by which atoms, molecules, or ions are taken up from the soil solution or soil atmosphere and retained on the surfaces of solids by chemical or physical binding. (Glossary of Soil Science Terms, 2013) advance ratio [AR]: Ratio of the time for the water to reach the end of the field to the total set time for an irrigation set on a furrow irrigation system. The ratio should be less than 0.5 to have a good distribution uniformity. (Burt, Surface Irrigation) advance time {min, s}: Time required for a given stream of irrigation water to move from the upper end of a field to the lower end. (ASAE, 2007) aeration soil: A diffusion process by which air in soil pore spaces is replaced by air from the atmosphere, replenishing soil oxygen and removing excess soil carbon dioxide. aeration capacity: Volume fraction of air-filled pores in a soil at field capacity. (Hess, 1999) aeration water: A process by which oxygen from the atmosphere is added to water bodies or flowing water. (ASAE, 2007) aggregate: Group of primary soil particles that cohere to each other more strongly than to other surrounding particles. (Glossary of Soil Science Terms, 2013) IA Glossary of Terms — page 1 air gap: See backflow prevention device. alfalfa valve: Outlet valve attached to the top of a pipeline riser with an opening equal in diameter to the inside diameter of the riser pipe and an adjustable lid or cover to control water flow… (ASAE, 2007) algicide: Substance that will kill or control algal growth. (ASAE S526.3 SSP, 2007) alkaline (alkali) soils: Soil with pH greater than 7.0. (Glossary of Soil Science Terms, 2013) allowable depletion [AD] {in., mm}: • Portion of plant available water that is allowed for plant use prior to irrigation based on plant and management considerations. • That part of soil moisture stored in the plant root zone managed for use by plants, usually expressed as equivalent depth of water in acre inches per acre, or inches. (NRCS) • Sometimes referred to as allowable soil depletion or allowable soil water depletion. allowable stress factor: See coefficient. allowable voltage loss [AVL] {volts}: Voltage loss in a circuit or portion of a circuit which, if not exceeded, will result in the electrical device working correctly. alternate set irrigation: See irrigation systems. alternate side irrigation: See irrigation systems. alternating current [AC]: Current in which the flow of electrons in a circuit flow in one direction and then in the reverse direction. (Principles of Irrigation, 2012) ampere {ampere or amp}: Unit of electrical current. The unit is used to specify the movement of electrical charge per unit time through a conductor. (Principles of Irrigation, 2012) anion: An atom or atomic group that is negatively charged because of a gain in electrons. (Glossary of Soil Science Terms, 2013) application efficiency: See efficiency. application efficiency of lower quarter: See efficiency. application efficiency low half: See efficiency. application rate {in./h}: Rate that water is applied to a given area. Usually expressed in units of depth per time. See also precipitation rate. (ASAE, 2007) application time, set time {min, h}: Amount of time that water is applied to an irrigation set. (ASAE, 2007} aquiclude: Underground geologic formation that neither yields nor allows the passage of an appreciable quantity of water, although it may be saturated with water itself. (ASAE, 2007) aquifer: A geologic formation that holds and yields useable amounts of water. Aquifers can be classified as confined or unconfined. (ASAE, 2007) aquitard: Underground geologic formation that is slightly permeable and yields inappreciable amounts of water when compared to an aquifer. (ASAE, 2007) arc {degrees}: Portion of a full circle (360 degrees) covered by a part-circle sprinkler. arid climate: See climate. artesian aquifer: Aquifer that contains water under pressure as a result of hydrostatic head. For artesian conditions to exist, an aquifer must be overlain by a confining material or aquiclude and receive a supply of water. The free water surface stands at a higher elevation than the top confining layer. (ASAE, 2007) atmospheric pressure [Pa] {psi, ft water, atmospheres, kPa}: Absolute pressure measured at any location. Standard atmospheric pressure at sea level is defined as 14.7 psi or 34.0 feet of water. IA Glossary of Terms — page 2 atmospheric vacuum breaker: See backflow prevention device. automatic drain valve: See valve. available soil moisture [ASM] {in., mm}: Difference at any given time between the actual soil moisture content in the root zone soil and the permanent wilting point. (On-farm Committee, 1979) available soil moisture capacity [AMC]: See available water. available water [AW] {%, in./ft, mm/mm}: The amount of water released between in situ field capacity and the permanent wilting point. See also available water holding capacity. (Principles of Irrigation, 2012 and Glossary of Soil Science Terms, 2013). available water holding capacity [AWHC] {in./ft, mm/mm}: Preferred term is available water. available water storage capacity [AWSC] {in./ft, mm/mm}: Preferred term is available water. average annual precipitation {in., mm}: Long-term historic (generally 30 years or more) arithmetic mean of precipitation (rain, snow, dew) received by an area. (NRCS) AVB: See atmospheric vacuum breaker under backflow prevention devices. B backflow: Any unwanted flow of used or nonpotable water or substance from any domestic, industrial or institutional piping system into the pure, potable water distribution system. The direction of flow under these conditions is in the reverse direction from that intended by the system and normally assumed by the owner of the system. (USC, 1998) backflow prevention device [BPD]: Safety device which prevents the flow of water from the water distribution system back to the water source. (ASAE, 2007) • air gap: Physical separation of the supply pipe by at least two pipe diameters (never less than 1 inch) vertically above the overflow rim of the receiving vessel. In this case, line pressure is lost. Therefore, a booster pump is usually needed downstream, unless the flow of the water by gravity is sufficient for the water use. With an air gap there is no direct connection between the supply main and the equipment. An air gap may be used to protect against a contaminant or a pollutant and will protect against both backsiphonage and backpressure. An air gap is the only acceptable means of protecting against lethal hazards. (USC, 1998) • atmospheric vacuum breaker [AVB]: Backflow device configured with a single moving part, a float, which moves up or down to allow atmospheric air into the piping system. (USC, 1998) • double check assembly [DC]: Two internally loaded, independently operating check valves together with tightly closing resilient seated shutoff valves upstream and downstream of the check valves. Additionally, there are resilient seated test cocks for testing of the assembly. The DC may be used to protect against a pollutant only. However, this assembly is suitable for protection against either backsiphonage or backpressure. (USC, 1998) • pressure vacuum breaker [PVB]: Backflow device configured with a spring-loaded float and an independent spring-loaded check valve. (Rochester, 1995) • reduced pressure principle assembly [RP, RPA, RPZ]: Consists of two internally loaded independently operating check valves and a mechanically independent, hydraulically dependent relief valve located between the check valves. This relief valve is designed to maintain a zone of reduced pressure between the two check valves at all times. The RP also contains